Aluminum coated with aluminum benzoate, and method and composition for making same



Sept. 1, 1959 w. H. Ross, JR 2,901,821 ALUMINUM COATED WITH ALUMINUM BENZOATE, AND METHOD AND COMPOSITION FOR MAKING SAME.

Filed Nov. 18, 1955 AL UM/NUM EENZOA TE COA TIA/G ALUMINUM ARTICLE INVEN TOR. mlfurafifiass J};

Faun P ATTORNEYS.

UnitedStates Patent ALUMINUM COATED WITH ALUMINUM BENZO- ATE, AND METHOD AND COIVIPOSITION FOR MAKING SAME Wilford H. Ross, J12, Detroit, Mich., assignor to Detrex Chemical Industries, Inc., Detroit, Mich.,, a corporation of Michigan "1 Application November 18, 1955, Serial No.1562,368 11 Claims. c1. 29-495 7 This invention relates to the application of protective coatings to aluminum or aluminum alloys, and more particularly relates to compositions and methods for inhibiting an aluminum surface against atmospheric oxidation. More particularly, this invention relates to the removal of oxide from an aluminum surface combined with the formation on such surface of a tightly adherent coating which resists atmospheric oxidation and which does not interfere with the welding of the metal.

It is an object of this invention to provide a coating. which has the advantageous characteristics just referred to. Still another object is to provide a method of applying such a coating to aluminum or its alloys. Still another object is to provide a method of removing an oxide film from an aluminum surface and for also applying to. such surface a chemical coating which protects the aluminum against atmospheric oxidation.

In the fabrication of aluminum articles, it is conventional to assemble prefabricated aluminum pieces by welding. However, aluminum has a much higher electrical conductivity than most of the other weldable metals. This necessitates the use of Welders of high capacity which are capable of supplying the high current necessary for Welding aluminum.

Aluminum combines almost instantaneously with atmospheric oxygen to form a crystalline aluminum oxide on the aluminum surface which has both a high and erratic electrical resistance. This in turn affects the amount of heat produced at the weld resulting in wide variation in weld strengths unless this oxide is removed prior to welding. This disadvantage is particularly felt when the aluminumvis to be spot welded.

It is accordingly necessary to clean and deoxidize the. aluminum surface to produce welds of consistent high strength, good appearance, minimum indentation, and

. maximum reliability. .Conventional production practice is to clean the aluminum article in a suitable cleaning medium, usually an inhibited-alkaline. cleaner, and then mechanically or chemically to deoxidize the surface. This resultsin a surface. having a contact resistivity of 20 to 1OQ microhms.

Conventional chemical deoxidizers are selected from the class of strong acids; they usually consist of sulfuric acid, chrornic acid, nitric acid or phosphoric acid or a combination of these acids, or their sodium salts suitably acidified to a low pH range or high acid content. These strongly acid chemicals, however, not only remove the oxide layer but also attack theunderlying aluminum. In attacking the aluminum, a second film is produced on the surface which sometimes has as'high an electrical resist ance as the'originaloxide' layer. herefore, time of exposure to such treatment is critical. 3 I I A numberof proprietary products are offered the trade basic aluminum surface. Included arnon'g such inhibitors are 'hydrofluosilicic,'jhydrofluorioor fiuoboric acids or other fiuorine contaiiiin'g materials which presumably react with any"freedaluininumfto form insoluble'aluminum silicofluoride,-fluoride, etc., Which-act as a passivating layer to further attack. These compounds have the t, 2 disadvantage, however, of increasing contact resistance and affecting the alloying character of the nugget or Weld sincethey are not volatilized at theweld. A further disadvantage of the strong acid deoxidizers is that special, expensive articles of equipment, including corrosion-resistant tanks, are necessary. Moreover, personnel hazards are involved in the use of strongly acid chemicals. It is" accordingly another object of this invention to provide a method as discussed above using a solution which operates at a relatively high pH range, avoiding personnel hazards and also costly equipment.

Another object is to provide a method and solution which remove the-crystalline oxide from an aluminum surface without seriously attacking or etching the underlying aluminum surface.

Still another object is to inhibit the treating solution so that ifany free aluminum goes into solution, it immediately reacts to form a passivating layer on the surface of the aluminum.

"Yet another object ofthis invention is to provide a coating for.,aluminum or its alloys which decomposes at a welding-temperature with the production of gaseous.

products and does not affect the alloying nature of the weld. v

Other objects and advantages of this invention will appear in further detail hereinafter, and in the drawing. The drawing represents a perspective view of an alumi-,

numarticle, in accordance with this invention, having an aluminum benzoate coating thereon. The drawing is essentially a schematic representation of the surface coating, which is extremely thin and does not therefore lend itself 'to accurate illustration. The surface markings in the drawing are intended to illustrate a general roughness of the surface area, and are not intended to suggest that thealuminum benzoate coating consists of specks or the like, or that such coating is essentially discontinuous in nature. The aluminum benzoate coating is fine grained and covers the entire aluminum surface.

It has been found that the foregoing objects are obtained by reacting the aluminum surface with benzenoid carboxylic acid. This ,is preferably but not necessarily Wetting agent attained by providing a mixture of acid phosphatesbuffercd to apH range of 4.4 to 5.5 containing a small percentage of benzenoid. carboxylic. acid. One example. of a suitable and preferred composition of matter is; 0.. 3 f Percent by weight Monosodium phosphate 91.8

Disodium phosphate 3.7 Benzoic acid 2.5

'(iso octyl, phenoxypolyoxyethylene ethanol) The following approximate ranges of proportions are applicable in thepractice of this invention: 7

I I Percent by Weight Alkali metal dihydrogen phosphate 85.0 to95.0 Alkali metal monohydrogen phosphate 2.5 to 5.0 Benzoic acid 0.25 to 10.0'

However, under some conditions, benzoic acid may be used independently of the acid phosphate chemicals.

, Thev Wetting agent, which may be varied considerably in form or chemical composition, or even entirely eliminated assists in Wetting the surface of the metal or oxide and facilitating chemical reactivity. I p It is important to provide a solution which is buffered which contain inhibitors to this subsequent attack on the :2; in thepH range 4.4-5.5. This can be accomplished by to a pH range of 4.4 to 5.5.,

It has been further discovered that the inclusion fof soluble alkali metal or ammonium chroma-tes, dichrofied'by way of example only:

Percent byweight Monosodium phosphate. 82.6 Disodium phosphate 2.2 Sodiumdichromate 13,.7 Benzoic acid -1.5.

The following approximate. ranges of proportions are applicable in the practice of this invention whenchromate is used:

Percentbyweight Alkali metal dihydrogen phosphate.... 7 5.0.to 90.0. Alkali metal monohydrogen,phosphate.. 1.5.to,4.0. Chromate (or chromic acid) as above 5.0.t 15.0. Benzoic acid 0.25 to 10.0 Similarly, surface active agents may be added to the foregoing composition to facilitate wetting. composition may likewise be duplicated by suitable neutralization of phosphoric acid with, alkali metal hydroxide or mixtures of potassiumor other alkali salts.

In use, these compositions are dissolved in water" at a concentration of preferably from 8" to 16' ounces total chemicals per gallon of water, the being more desirable. plished byheating to Operating temperatures 180-200 F.

The aluminum part to be deoxid'ized and coated is usually pro-cleaned in an inhibited alkali solution followed by a water rinse, orby degreasing-withtrichloroethylene followed by an alkali cleaning treatment. The part is then immersed in the solution ofthe deoxidizer as, heretofore defined, for about to 10 minutes, and then water rinsed. Aluminum oraluminum alloy processed in this way has. none of the objectionable etched appearance common with str ,nger acid deoxidizers ,but remains bright. The period of immersion in the bath should be limited, however, since immersion for an excessive period tends to produce a light brown or black discoloration, depending upon the alloy; This discoloration also indicates an excessive surface contact resistance. Theappropriate immersion. timecan readily be. obtained. by visual observation.

Parts processed in the manner described herein have been found to resist atmospheric oxidation, for longer periods of time-than untreated aluminum...

Although this invention has, been described with particular reference to the application, of. a. coating to aluminum, it will be appreciated that the term aluminum, as referred to herein, is intended to cover either wrought, cast, extruded, or any other form of aluminum, and is also intended to cover the various alloys of aluminum.

While this invention has particular advantages in connection with the preparation of aluminum for electric spot-welding, it will be appreciated that the coating as applied in accordance with this invention has a wide variety of other uses.

Various modifications may be made in the coating solution and in the method of applying the coating to the metal, including substitution of equivalents for the specific chemicals and method steps described in the foregoing specification, and further including: the useof certain features of the invention independently of otherfeatures. All such modifications may be made without departing from the spirit or scope of invention, as defined in the appended claims, a

Solution is preferably accomtemperatures overl40-1,60' F.

of the bathare in the range of? latter figure usually containing dihydrogen phosphate Having-thus described my invention, I" claim:

1. Method of welding aluminum or alloys thereof comprising chemically coating the aluminum surface with a thin film of aluminum benzoate, and then heating the aluminum and aluminum benzoate coating to welding temperature.

2. Method of coating or itsyalloys comprising immersing the aluminum in a. dilute water solution coating chemicals and benzoic acid, said benzoic acid being present in a solution having a pH of. about 4.4 to 5.5. and: in a. substantial quantity to react" with said aluminum toform an aluminum benzoate coating.

3., Method of welding or an alloyv thereof having aluminum oxide on the surface thereof comprising chemically coating-thesurfaceof said. aluminum with aluminum benzoate, and heating the aluminum to welding temperature thereby decomposing the benzoate to form gaseous productswhile the aluminum is welded.

4. Methodof welding aluminum or an aluminum alloy, having aluminum oxide on the surface thereof comprising etching the aluminumwith a benzoic acid solution having a pH of about 4.4 to 5.5, thereby dissolving thealuminum oxide and forming an integral coatingof aluminum benzoate on the surface of said aluminum, and heating the aluminum to welding temperature thereby decomposingthe aluminum benzoate as the aluminum is welded. v

5'. An article having an aluminum surface covered by a coating formed in situ on said surface by contacting and reacting said aluminum surface with a dilute aqueous solution of benzoic acid containing a bufieradjusting the solution pH toabout a substantially insoluble benzoate ofaluminum.

-6. An article having a surface of aluminum coated with a fine grained, strongly adherent coating formed integrally in situ on said surface by contacting and reacting said aluminum surface with a dilute aqueous solution ofbenzoic acid containing a buffer adjusting the solution pH to about 4.4 to 5.5, at least the major portion of said coating being benzoate and being substantially free of aluminum oxide.

7. A composition of matter for coating aluminum oritsalloys to protect the metal against atmospheric oxidation prior to welding consistingessentially'ofan acidic aqueous solution of benzoic acid and alkali metal dihydrogen phosphate, said solution having a pH- of about 4.4 to 5.5, and said benzoate being present in a substantial quantity to react with aluminum to form an aluminum benzoate coating.

8; A composition of matter its alloys to protect the metal against. atmospheric oxidation prior to. welding consisting essentially of an acidic aqueous solution of benzoic acid, alkali metal dihydrogen phosphate, and chromium, compound selected from the group, consisting of the chromates and dichromates and chromic acid, said solution havinga pH of about 4.4 to 5.5, and said benzoic acid comprising; about 0.25 to 10.0% by weight oft-he combinedweight of benzoic acid 10. Composition, of matter. for coating aluminum consisting essentially of: a a r Percent by weight Benzoic, acid: i...,...; 0.25 to 10.0 Alkali metal dihydrogen phosphate-.- 75.0-to 90.0 Alkali metal mouohydrogen hosphate Chromate compound selected frOmlhfi-gl'oup 7 consisting ofjalkali metal chromates and dichromates andehromic acid' ';..1 5.0 to 15.0

4.4 to 515, and comprising for coating aluminum or 5 11. Method of applying a coating to aluminum or an alloy thereof comprising contacting and reacting the aluminum with a dilute aqueous solution of benzoic acid containing a bufier adjusting the solution pH to about 4.4 to 5.5.

References Cited in the file of this patent UNITED STATES PATENTS 1,911,537 Tanner May 30, 1933 

1. METHOD OF WELDING ALUMINUM OR ALLOYS THEREOF COMPRISING CHEMICALLY COATING THE ALUMINUM SURFACE WITH A THIN FILM OF ALUMINUM BENZOATE, AND THEN HEATING THE ALUMINUM AND ALUMINUM BENZOATE COATING TO WELDING TEMPERATURE.
 7. A COMPOSITION OF A MATTER FRO COATING ALUMINUM OR ITS ALLOYS TO PROTECT THE METAL AGAINST ATMOSPHERIC OXIDATION PRIOR TO WELDING CONSISTINGESSENTALLY OF AN ACIDIC AQUEOUS SOLUTION OF BENZOIC ACID AND ALKALI MSERAL DIHYDROGEN PHOSPHATE, SAID SOLUTION HVINGA PH OF ABOUT 4.4 TO 5.5, AND SAID BENZOATE BEING PRESENT INA SUBSTANTIAL QUANTITY TO REACT WITH ALUMINUM TO FORM AA ALUMINUM BENZOATE COATING. 